Switching power supplies are popular for high power applications because of the high efficiency and small area/volume required. Buck converters in particular are well suited to providing the high current at low voltages needed by high performance digital integrated circuits such as microprocessors, graphics processors, and network processors. For examples a buck converter is often used to step down a DC voltage (typically referred to as the input voltage) to a lower DC voltage (typically referred to as the output voltage). Since the power stage is fully switched (i.e., the power MOSFET is fully off or on), there is very little loss in the power stage and the converter power efficiency is very high.
The inductance value in a buck converter is typically chosen to be sufficiently high such that the ripple current is at an acceptable level. The high inductance level, however, limits the ability of the regulator to quickly change its output current. This tradeoff between ripple current and output current slew rate transient performance becomes a limiting effect on the regulator transient and steady state performance.
The tradeoff can be mitigated by replacing the inductors in a multi-phase regulator with mutually coupled inductors. Coupled inductors effectively allow the ripple current in each inductor to be reduced by using the coupling current from the other phases to cancel out the current in each phase. The reduction in ripple current allows lower value inductors to be used, which improves the output current slew rate and the regulator's transient performance.
In addition, many power supplies operate in conjunction with current sensors to monitor the current in the power supply and the load. Referring to FIG. 1, one common lossless current sensing technique is known as inductor DCR (DC resistance) current sense. For simplification, FIG. 1 illustrates the widely used “output reference” equivalent form, where the output voltage is shown as ground, and the voltage applied across the inductor (Vin−Vout) is shown as V1. By adding the RC network in parallel with the inductor L with its parasitic DC resistance r and matching the time constants, the voltage across the capacitor C is proportional to the current through the inductor multiplied by the DCR. A voltage amplifier connected to the PWM controller generates the desired signal representing the current through the inductor. This method is popular because the DCR of inductors is well controlled and characterized for tolerance and temperature variation, resulting in accurate current sensing.